Market sizing is one of the most abused analytical exercises in B2B strategy. Most TAM numbers are either wildly inflated (top-down from analyst reports that define "market" loosely) or arbitrarily precise (bottom-up calculations with confident presentation but shaky inputs). A well-constructed TAM analysis does something more useful than either: it tells you where to focus, how to prioritise, and whether your current GTM motion is correctly calibrated to the opportunity.
TAM, SAM, and SOM defined
The three market sizing concepts serve different strategic purposes. Conflating them is the most common TAM analysis mistake.
Most B2B companies present their TAM to investors and use their SOM for internal planning. The number that matters most for GTM decisions is the SAM — and it is the least frequently calculated with any rigour.
Three methods for calculating TAM
Each method has different data requirements, accuracy levels, and appropriate use cases. For most B2B companies, a bottom-up calculation is the most credible and strategically useful.
Start from a published industry market size (from Gartner, IDC, Forrester, or similar) and work down by geography, segment, and ACV. Fast and credible to investors who recognise the source. Unreliable for operational planning because analyst "market size" definitions rarely match your actual ICP. Use for investor decks, not GTM decisions.
Count the number of companies that match your ICP criteria (using LinkedIn Sales Navigator, Apollo, Cognism, or Companies House data), then multiply by your average contract value. This is the most operationally useful method because it produces an actual list of companies, not an abstraction. It also forces you to define your ICP precisely — which has GTM value beyond market sizing.
Estimate the value your solution creates for customers (cost savings, revenue uplift, productivity gain), apply a pricing assumption (typically 10-30% of value created), and multiply by the number of potential customers. Useful for early-stage products without established pricing, or for justifying premium pricing to investors. Rarely matches bottom-up calculations.
Building a bottom-up TAM: worked example
This worked example uses a hypothetical B2B SaaS company targeting HR Directors at UK firms with 200-2,000 employees. Walk through the same framework with your own ICP criteria and data sources.
Why most B2B TAM numbers are wrong
The TAM errors that matter aren't rounding errors in the spreadsheet — they're category errors in how the problem is framed.
A company selling ABM software to enterprise B2B firms does not have a TAM equal to "the global marketing software market." The TAM is the universe of companies that match the ICP who could realistically buy the product. The difference is often 100× or more — and it matters enormously for GTM planning.
Presenting TAM in an investor deck is appropriate. Using TAM to set sales targets is not. A UK SaaS company with a London-based sales team and no channel partnerships cannot realistically address a global TAM. SAM is what determines headcount, quota, and marketing budget.
Even within your SAM, only 3-5% of companies are in active buying mode at any given time. The rest are either not-yet-in-market or locked into competitor contracts. This doesn't reduce your TAM, but it fundamentally changes how you structure your GTM — you need a 12-18 month demand generation motion to reach the 95%, not just a lead capture mechanism for the 5%.
Markets change. ICP companies grow, shrink, merge, and enter new sectors. A TAM calculated in 2022 using 2021 company data may be significantly different from today's reality — particularly after economic disruption. Rebuild the bottom-up count annually using fresh data pulls.
What market sizing tells you about your GTM
A well-constructed TAM analysis answers strategic questions that gut feel cannot. Here are the four most valuable decisions it informs.
- Sales motion selection: A SAM of under £50M with high ACV (£50k+) suits an enterprise sales motion with a small, focused AE team. A SAM of £500M+ with low ACV (£5-15k) suits a product-led or inside sales motion. The motion should match the market, not the founders' preference.
- ICP prioritisation: If your bottom-up count produces 20,000 companies but your sales capacity can work 200 accounts per year, you need a sub-ICP prioritisation framework — typically using firmographic fit, intent data, and account-level engagement signals to identify the 200 highest-probability accounts.
- Marketing budget calibration: A common rule of thumb is to invest 5-10% of your SAM revenue target in marketing annually. A £50M SAM with a 5% share target (£2.5M revenue) suggests a £125k-£250k annual marketing budget. Significantly below that level produces insufficient reach; significantly above it suggests inefficiency.
- Geographic expansion sequencing: If you have a strong position in one geography (say, UK), your SAM analysis for the next market (DACH, Nordics, US) tells you whether expansion economics are attractive before you invest in a new sales hire or country office.
A structured spreadsheet and methodology for calculating your Total Addressable Market, Serviceable Addressable Market, and Serviceable Obtainable Market — with UK B2B data sources and worked examples.
B2B TAM Analysis Workbook
The complete frameworks, checklists, and templates from this guide in a single PDF.
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